Instruments, which are called "peak-flow-meters", are known in general and are used by a treating physician in the therapeutic monitoring of the lung function where the expiratory peak exhalation speed plays a significant role. An instrument of the above described type is known, for instance, from DE 24 33 994 Al in which a rod installed in the case is permanently fastened in a case wall with one end, and in which the receptacle for a mouth piece, which is plugged onto the air inlet opening, is screwed onto the other end of the rod. An instrument of the above mentioned type is known from U.S. Pat. No. 4,944,306, in which the two halves of the case and the receptacle for a mouth piece are firmly welded to each other and the rod is permanently installed inside the case. Instruments of the above mentioned type are further known, in which the halves of the case are screwed onto each other, for example, and the receptacle for the mouth piece is fastened on the inlet opening with fixing devices which cannot be detached manually.
Assembly of all of these known instruments is difficult and cumbersome because of their design, and in some instances only possible with tools. Furthermore, also due to their design, all of these known instruments can only be cleaned with difficulty or not at all, since they can either not be disassembled at all or only very awkwardly with tools. A complete disassembly of the instruments, however, is absolutely necessary for the thorough and complete cleaning of the same, since the warm and humid exhalation air blown into the instrument creates extremely favorable conditions for the growth of bacteria and fungi inside the instruments, which can only be removed by a thorough cleaning of the individual parts.